The advent of global communications networks such as the Internet has presented commercial opportunities for reaching vast numbers of potential customers. Electronic messaging, and particularly electronic mail (“email”), is becoming increasingly pervasive as a means for disseminating unwanted advertisements and promotions (also denoted as “spam”) to network users.
The Radicati Group, Inc., a consulting and market research firm, estimates that as of August 2002, two billion junk e-mail messages are sent each day—this number is expected to triple every two years. Individuals and entities (e.g., businesses, government agencies) are becoming increasingly inconvenienced and oftentimes offended by junk messages. As such, spam is now or soon will become a major threat to trustworthy computing.
A key technique utilized to thwart spam is employment of filtering systems/methodologies. One proven filtering technique is based upon a machine learning approach—machine learning filters assign to an incoming message a probability that the message is spam. In this approach, features typically are extracted from two classes of example messages (e.g., spam and non-spam messages), and a learning filter is applied to discriminate probabilistically between the two classes. Such conventional learning filters commonly referred to as “content-based filters,” typically scan messages for particular dictionary words or phrases that are more characteristic of spam. However, with the onslaught of such spam filtering techniques, many spammers have thought of ways to disguise their identities to avoid and/or bypass spam filters. Thus, conventional content-based and even adaptive filters may become ineffective in recognizing and blocking disguised spam messages.